Wireless communication systems are continually striving to increase the data bandwidth so that information can quickly be exchanged between devices coupled to the communication system. Some of the parameters that limit the data bandwidth available to devices include the spectral bandwidth allocated to the devices and the quality of the channel linking the devices.
Wireless communication systems compensate for the various constraints on data bandwidth using a variety of techniques. A wireless communication system may incorporate multiple encoding techniques, and may select an encoding technique based on a data rate supported by a channel. In such a system, the communicating devices may negotiate a data rate based on the capabilities of the channel. Such a communication system may be advantageous for multiple point-to-point links, but may be less than ideal in a distributed broadcast system where a single transmitter provides substantially the same data to multiple receivers.
Wireless communication systems may incorporate hierarchical modulation, also referred to as layered modulation, where multiple data streams are simultaneously transmitted across a hierarchy of data layers. The multiple data streams can include a base layer that is a robust communication link capable of successful reception in nearly all receiver operating conditions. The multiple data streams can also include an enhancement layer that is broadcast at a data rate that is lower, the same, or at a higher data rate than the base layer. The communications over the enhancement layer may require a higher signal quality at the receiver compared to the base layer. Therefore, the enhancement layer may be more sensitive to variations in the quality of the channel.
The receiver is typically ensured the ability to communicate at the base level, and can typically demodulate data on the base layer. In channel conditions sufficient to support the enhancement layer, the receiver is also able to demodulate additional data modulated on the enhancement layer to provide a higher quality of service or to provide additional data bandwidth.
The use of hierarchically modulated signals substantially complicates the receiver operation. Further, the receiver may be a portable receiver that has limited power capacity or limited processing capabilities. The complications to the receiver arising from the incorporation of layered modulation operate in contrast to efforts to reduce the size, power consumption, and cost of a receiver.